Shear diffusion and the spread of oil in the surface layers of the North Sea

Summary

Controlled releases of oil and dye have been made in the southern North Sea, and the slicks observed during the six hours following release compared with the results of a numerical model. The model included the vertical and lateral shears associated with the tidal streams, the vertical shears due to Stokes drift and a logarithmic wind-driven velocity profile. A patch of oil was modelled as a distribution of droplets whose individual buoyancies depended on droplet size, and the turbulence in the water column was represented by a three-dimensional random walk process. The elongation and orientation of the predicted slicks were in good agreement with observations, suggesting that near-surface vertical shears are dominant in determining the spreading rate of a slick soon after release. The buoyancy of the droplets effectively limits the depth through which the oil can mix, consequently the oil spreads in a manner that represents a combination of first and second stage shear diffusion processes. Numerical results showed that the major axis of a slick should elongate linearly with time, while the width of a slick will grow ast0.5. The model correctly predicted the occurrence of thicker oil towards the leading edge of a slick, and the alignment of slicks in the direction of the wind.